This invention relates to positive displacement scroll pumps and in particular to additional discharge means affixed to the scroll impeller vanes and end plates.
A detailed description of scroll pump operation may be found in my copending U.S. application Ser. No. 891,963, filed Mar. 30, 1978. In general, however, scroll pumps operate with a relative orbiting motion between the meshed scroll vanes, forming crescent-shaped pockets between the vanes. The scroll vanes are not free to rotate relative to each other as one scroll member orbits about the other. In axial discharge pumps of the present type, the volume of the pocket continually decreases during the pumping cycle. This decreasing volume characteristic is useful in fluid compressors, in which the volume of the trapped pocket is reduced until the required discharge pressure is attained. Discharge is achieved by modifying one of the scroll vanes to allow the fluid to escape into the discharge area. In actual operation, the fluid in the trapped pocket continues to be compressed during the time that the discharge port is opening. This is a minor problem when the fluid is a gas, but when the fluid is a liquid the problems are significant. When pumping a liquid the nominal compression ratio has to be unity, but, due to the slowly opening discharge port and the continuing reduction in trapped pocket volume, severe overpressurization occurs. This results in ineffeciencies and probable mechanical damage to the pump due to the additional loading.
Prior art devices have attempted to overcome this problem by modifying the geometry of the scroll vanes to maintain a constant trapped pocket volume during the discharge period until the discharge port is fully opened. This method alleviates the overpressurization, but the required scroll vanes are difficult to fabricate.